Near-field communication (NFC) tags optimized for high performance NFC and wireless power reception with small antennas

ABSTRACT

A device for near-field communication (NFC) and wireless power reception (WPR) using a magnetic field. The device has an antenna resonant circuit. The antenna resonant circuit includes an antenna for magnetic flux of the magnetic field to flow therethrough, to thereby receive a NFC signal during the NFC and receive wireless power during the WPR, and a multi-Q antenna matching circuit configured to adjust an impedance of the antenna to thereby adjust a quality factor (Q-factor) of the antenna resonant circuit. The multi-Q antenna matching circuit is configured to switch between a high-Q mode for the WPR and a low-Q mode for the NFC, based on whether strength of the magnetic field is larger than a predetermined threshold. The device may also include two separate antenna resonant circuits, of which the Q-factors are respectively no higher than 25 and no lower than 50.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to wireless communication andwireless power transfer (WPT), and more specifically to Near-fieldCommunication (NFC) tags optimized for high performance NFC and wirelesspower reception with small antennas.

2. Background Information

NFC technology became a popular short-distant secure communicationapproach in recent years. NFC leverages the fast decaying magnetic fieldas its communication medium, and realizes a short communication distanceof merely a few centimeters, which grants high security and usability.

As shown in FIG. 1, there exist two types of typical NFC interfaces,which are NFC Reader 101 and NFC Tag 102. Near-field communication isusually conducted between these two interfaces. NFC is a half-duplexcommunication system, which contains two communication links, i.e.,Reader->Tag link 103 and Tag->Reader link 105.

Reader->Tag link 103: NFC Reader 101 generates an oscillating MagneticField 104 with a center frequency of 13.56 MHz as information carrier.NFC Reader 101 maintains the presence of the Magnetic Field 104throughout the entire communication process, regardless of the activecommunication link. The carrier is modulated by NFC Reader 101 totransmit information for the Reader->Tag link 103. When the NFC Tag 102is in the vicinity of the magnetic field 104, it collects the energycarried by the field, and demodulates the information superimposed onthe field to retrieve information. To ensure sufficient bandwidth, theantenna quality factor of the NFC Tag 102 is sufficiently low (<30).

Tag->Reader link 105: Conventional NFC tag interfaces are passiveinterfaces that do not emit any radio frequency (RF) energy. They relyon the passive load modulation on Magnetic Field 104 for datatransmission. Specifically, passive NFC tag interfaces modify theimpedance of the load that connects to the antenna for transmission. Thevariation of the load impedance varies the strength of the MagneticField 104. This results in controlled variation of the current flowingthrough the reader's antenna, which can be measured to demodulate theinformation.

Because the NFC reader interface emits high power in communication, itis usually adopted by devices with abundant energy, such as smartphones,tablets, and POS terminals. On the other hand, NFC tag interfaces areusually employed by low power devices, like smart cards and wearabledevices.

NFC Tag 102 can be configured to collect the energy carried by theoscillating magnetic field 104, to power the interface itself and otherconnected devices. This is called as “NFC energy harvesting,” which iswidely utilized on applications like smart cards and smart tags.

FIG. 2 shows the typical architecture of conventional passive NFC taginterfaces. Antenna 201 is comprised of one or many loops of conductivewires, which receive the energy and the modulated information carried bythe oscillating magnetic field. Antenna match circuit 202 transforms theimpedance of Antenna 201 to a suitable value. Demodulator 203demodulates the received signal and recovers the original information.Load Modulator 205 modulates the impedance of the load connecting to theAntenna 201 to transmit information. Data Interface 204 is connectedwith external components like microcontrollers (MCU) via a data bus,which is used for exchanging data and configuration. Rectifier andRegulator 206 converts the received RF energy to regulated DC (directcurrent) energy that could be used for powering system components.

FIG. 3 shows the typical architecture of NFC reader interfaces. Antenna301 is comprised of one or many loops of conductive wires, whichgenerate oscillating magnetic field, transmit, and receive NFC signals.Antenna match circuit 302 transforms the impedance of Antenna 301 to asuitable value for improving efficiency. Modulator 304 modulates thesignal used for generating oscillating magnetic field according to thedata to be transmitted. Antenna Driver 303 amplifies the signalModulator 304 generated, and drives Antenna 301 via Antenna matchcircuit 302. To improve power efficiency, Antenna Driver 303 usually haslow output impedance. Demodulator 309 measures and tracks the strengthof the current flowing through Antenna 301, and demodulates thesuperimposed signal. MCU 307 manages the entire interface, and its tasksinclude: assembling and dissembling NFC frames, data integrityverification, data exchange via Data Interface 305, controlling andmanagement of on-chip components. Data Interface 305 is thecommunication interface between the NFC reader and external components,and is usually in the form of SPI (Serial Peripheral Interface), I²C(Inter-Integrated Circuit), or UART (universal asynchronousreceiver/transmitter). FIFO (First In First Out buffer) 306 serves as abidirectional buffer between Data Interface 305 and MCU 307. ClockSystem 308 generates the necessary clocks for the NFC reader interface,including the 13.56 MHz carrier frequency. On-chip Power Supply 310provides regulated power and reference for the NFC reader interface.

Conventional NFC system has two major disadvantages. First, passive NFCtag interface requires an antenna of a large size to realize areasonable communication distance. Due to the weak signal generated bypassive load modulation with a low-Q antenna, passive NFC tag interfacesmust use sufficiently large antennas to increase the coupling betweenthe antennas of NFC reader and passive tag interfaces. When the antennais too small, the low coupling results even weaker passive loadmodulation signal that cannot be correctly received by the NFC reader.Second, the low-Q antenna systems of NFC system lead to low wirelesstransfer efficiency, which only allows very little power to be collectedby the passive tag NFC interface (10 mW to 20 mW). Such limited powercan only support very simple operations, like read/program internalmemory.

Many current and most next-generation smart devices like wearabledevices, smart cards, and Internet-of-Things (IoT) have smallform-factors that cannot afford large NFC antennas. However, smallantennas significantly limit the performance and reliability of NFC,resulting in very short communication distance and unreliableconnection.

Many current and next-generation NFC applications such as wearabledevices, smart cards, and smart sensors require significantly higher NFCenergy harvesting capacity than current NFC products could provide, dueto their sophisticated functions and high processing power. Theextremely limited NFC energy harvesting capability significantly limitsthe performance of these devices.

To solve the weak signal problem caused by small NFC antennas, currentmainstream solutions employ active modulation techniques to replacepassive load modulation on the NFC tag interfaces. Active modulationtechniques actively emit RF signals that do not rely on the carriersignal, a.k.a., the oscillating magnetic field. As active modulation canemit arbitrarily high power, small antennas can yield the similarcommunication performance as larger antennas. However, active modulationis not a perfect solution to this problem. First of all, since activemodulation generates RF signals when the carrier signal is stillpresent, it requires precise phase and frequency synchronization of thegenerated RF signal to the carrier frequency. This calls for complex PLL(phase-locked loop), antenna drivers, and phase tracking circuits, whichgreatly increase system cost and power consumption. Moreover, activemodulation technology cannot support NFC energy harvesting due to itsprinciple of operation. It requires external power to operate.Therefore, applications relying on NFC energy harvesting, like smartcards and smart sensors, are incompatible with the active modulationtechnology.

To mitigate the problem of limited NFC energy harvesting capability onNFC tag interfaces, current solutions actively decrease the powerconsumption of devices, so that the limited harvested power can stillsupport normal operation. These solutions include employing advanced IC(integrated circuit) manufacturing techniques (e.g., from 130 nm to 90nm process), increasing device sleep time, lowering device operatingfrequency, etc. However, these methods solve the problem at the expenseof cost or performance.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the invention relates to a near-field communication(NFC) reader for NFC and wireless power transfer. The NFC reader has anantenna resonant circuit that includes an antenna for transmitting andreceiving signals, a multi-Q antenna matching circuit for adjusting aquality factor (Q-factor) of the antenna resonant circuit, and anantenna driver for driving the antenna through the multi-Q antennamatching circuit. The NFC reader also includes a microcontroller (MCU)for controlling the multi-Q antenna matching circuit, the MCU beingconfigured to control the multi-Q antenna matching circuit to switchbetween a high-Q mode for the wireless power transfer and a low-Q modefor the NFC.

Another embodiment of the invention relates to a NFC tag for NFC andwireless power reception. The NFC tag includes an antenna resonantcircuit that has an antenna for transmitting and receiving signals, anda multi-Q antenna matching circuit for adjusting a Q-factor of theantenna resonant circuit. The multi-Q antenna matching circuit switchesbetween a high-Q mode for the wireless power reception and a low-Q modefor the NFC, based on whether field strength for the NFC is larger thana predetermined threshold.

Yet another embodiment of the invention relates to a NFC tag for NFC andwireless power reception. The NFC tag includes first and second resonantcircuits that are separate from each other. The first antenna resonantcircuit is configured to perform the NFC, and includes a first antennafor transmitting and receiving signals, and a first antenna matchingcircuit connected to the first antenna, a Q-factor of the first antennaresonant circuit being no higher than 25. The second antenna resonantcircuit is configured to perform the wireless power reception, andincludes a second antenna for transmitting and receiving the signals,and a second antenna matching circuit connected to the second antenna.The Q-factor of the second antenna resonant circuit is no lower than 50.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a typical NFC system.

FIG. 2 illustrates the architecture of a typical NFC tag.

FIG. 3 illustrates the architecture of a typical NFC reader.

FIG. 4 is an illustration of antenna coupling.

FIG. 5 is an equivalent antenna resonant circuit diagram of NFC systems.

FIG. 6 illustrates the architecture of an NFC reader optimized forworking with small antennas and wireless power transfer in oneembodiment of the invention.

FIG. 7 illustrates the architecture of the antenna resonant circuit ofthe NFC reader optimized for working with small antennas and wirelesspower transfer.

FIG. 8 is the state machine transitional chart of the NFC readeroptimized for working with small antennas and wireless power transfer.

FIG. 9 illustrates the architecture of a first NFC tag optimized forworking with small antennas and wireless power reception in oneembodiment of the invention.

FIG. 10 illustrates the architecture of the antenna resonant circuit ofthe first NFC tag.

FIG. 11 is the state machine transitional chart of the first NFC tag.

FIG. 12 illustrates the architecture of a second NFC tag optimized forworking with small antennas and wireless power reception in anotherembodiment of the invention.

FIG. 13 illustrates the architecture of the antenna resonant circuit ofthe second NFC tag.

FIG. 14 is the state machine transitional chart of the second NFC tag.

FIG. 15 is the state machine transitional chart of the first NFC tagwhen wireless power reception is not needed.

DETAILED DESCRIPTIONS OF THE INVENTION

The present invention relates to an NFC reader interface and a passiveNFC tag interface, which are specially optimized for working with smallantennas and NFC energy harvesting. The disclosed NFC reader and passivetag interfaces are fully compatible with current NFC standards,therefore they can work with any other NFC device.

Analysis of NFC Energy Harvesting Efficiency

NFC energy harvesting is a special case of inductive coupling wirelesspower transfer. For any inductive coupling wireless power transfersystem, the maximum energy transfer efficiency η_(max) can be expressedas:

$\begin{matrix}{\eta_{\max} = \frac{U^{2}}{\left( {1 + \sqrt{U^{2} + 1}} \right)^{2}}} & (1) \\{U = {k\sqrt{Q_{1}Q_{2}}}} & (2)\end{matrix}$where k is the coupling coefficient between the antennas of NFC readerand tag, Q₁ and Q₂ are the Q-factors of the reader's and the tag'santenna resonant circuits when oscillating at 13.56 MHz, respectively.Note that η_(max) is the maximum efficiency a wireless power transfersystem could reach. The actual efficiency also depends on systemsource-load impedance match.

The coupling coefficient reflects the degree of coupling between twoantennas. It can be viewed as the percentage of magnetic flux generatedby one antenna that passes through another antenna. Generally, thefurther apart are the antennas, the lower is the coupling coefficient,as shown in FIG. 4. Antenna 402 generates a magnetic field near Antenna404 and 405. Because Antenna 405 is closer to Antenna 402 than Antenna404, more magnetic flux 403 flows through Antenna 405 than throughAntenna 404. As a result, the coupling coefficient between Antenna 405and 402 is higher than that between Antenna 404 and 402. Moreover, thecoupling coefficient is also related to the difference between sizes ofthe two antennas. For example, even if Antenna 401 and 404 are placed atthe same distance from Antenna 402, a smaller amount of magnetic flux403 passes through Antenna 401 than through Antenna 404, due to itssmaller size.

Q-factor describes the frequency-selectivity and efficiency of a circuitat a given frequency, which could be calculated as:

$\begin{matrix}{Q = \frac{X}{R}} & (3)\end{matrix}$where X and R are the reactance and the resistance of the circuit,respectively. The higher the Q is, the more selective and higherefficient the circuit becomes, and vice versa. In the case of theantenna resonant circuit, the higher is the Q, the lower is the losswhen oscillating.

The above analysis indicates four factors that collaboratively determinethe wireless power transfer efficiency, which are the couplingcoefficient, the Q-factor of the antenna resonant circuit of the readerQ₁, the Q-factor of the antenna resonant circuit of the tag Q₂, and thedegree of impedance matching at the tag's end. Specifically,

Coupling coefficient k: From Equation (1) and (2), tighter couplingbetween antennas leads to higher wireless power transfer efficiency. Thecoupling coefficient is determined by the relative position, and thesize difference of the two antennas. It usually cannot be directlycontrolled by system designers, since it is much related to the natureof applications and the industrial design of the final product.

Q-factor of the antenna resonant circuit of the reader Q₁: According tothe above analysis, higher Q₁ leads to higher wireless power transferefficiency. Note that Q₁ is the Q-factor of the entire resonant circuit,which is collectively determined by the antenna Q-factor, the antennadriver ESR (Equivalent Series Resistance), and the loss of antennamatching circuit.

Q-factor of the antenna resonant circuit of the tag Q₂: According to theabove analysis, higher Q₂ leads to higher wireless power transferefficiency. Note that Q₂ is the Q-factor of the entire resonant circuit,which is collectively determined by the antenna Q-factor, and the lossof antenna matching circuit.

Impedance matching of NFC tag: k and Q determines the maximum wirelesspower transfer efficiency a system can reach, but the actual efficiencyis also determined by the degree of matching of the load impedance attag to the source when seeing into the antenna matching circuit. Preciseimpedance matching is usually difficult on most systems due to loadimpedance variations.

Analysis of Load Modulation when Using Small Antennas

For the majority of NFC systems, the bottleneck of NFC performance isthe performance of the Tag->Reader communication link. FIG. 5 shows atypical NFC system, which is comprised of an NFC reader interface (left)and an NFC tag interface (right). To simplify the analysis, both antennamatching circuits of the two interfaces are comprised of a single tuningcapacitor connected in series with the antenna. However, the analysisapplies to all forms of matching circuit topologies. Antenna Driver 511is an RF power amplifier, whose output impedance is represented asResistor 501. Antenna 504 on the reader is matched to the TuningCapacitor 502, which resonates at 13.56 MHz. Resistor 503 is thecombined resistance of the Antenna 504 and the Tuning Capacitor 502.Antenna 505 on the tag is matched to the Tuning Capacitor 507, whichresonates at 13.56 MHz. Resistor 506 is the combined internal resistorof the Antenna 505 and the Tuning Capacitor 507. RF Switch 508constitutes the load modulator used for data transmission on the tag,whose impedance is represented as Resistor 509. When the NFC taginterface is placed in vicinity of the oscillating magnetic fieldgenerated by the NFC reader interface, the tag interface can be viewedas a load to the reader interface. To be specific, NFC tag could berepresented as a resistor connected in series with the antenna of thereader, as illustrated as Resistor 510. This resistor is commonly called“Reflective Resistance.” The value of this resistor is determined bymany factors, including coupling coefficient between antennas, Q-factorsof both resonant circuits, load to the tag interface, and etc. However,for any given reader interface and tag interface, when their relativeposition is fixed (coupling coefficient and Q-factors are thus fixed),the value of the reflective resistance only varies with the load to thetag interface. The variation of the load will vary the value of thereflective resistance. Therefore, a NFC reader can measure the change ofthe current flowing through its antenna to receive the data transmittedby the tag interface. This is the principle of load modulation.

Apparently, greater variations of reflective resistor 510 during loadmodulation will lead to stronger current change on the antenna of theNFC reader interface, which creates higher signal strength. Theresistance of reflective resistor 510, when the antennas are resonant,can be expressed as:

${Z_{r} = \frac{\omega^{2}M^{2}}{R_{2} + R_{L}}},$where ω is the signal frequency, M is the mutual inductance of the twoantennas, R₂ and R_(L) are the resistance of and the load to the tag'santenna resonant circuit, respectively. Because ω, M, and R₂ areconstant during communication, the variation of Z_(r) can be onlygenerated by the change of R_(L). Apparently, when the resistance ofR_(L) is switching between 0 and infinity, Z_(r) has the highestvariation. The maximum and minimum values of Z_(r) can be expressed as:

${{{Max}\left( Z_{r} \right)} = \frac{\omega^{2}M^{2}}{R_{2}}},{{{when}\mspace{14mu} R_{\; L}} = 0}$${{{Min}\left( Z_{r} \right)} = {\frac{\omega^{2}M^{2}}{R_{2} + \infty} = 0}},{{{when}\mspace{14mu} R_{\; L}}->{+ \infty}}$

The signal strength generated by load modulation, H, can be written asthe ratio between the impedance variation caused by load modulation andthe maximum impedance on the reader's antenna resonant circuit:

$\begin{matrix}{H = {\frac{{{Max}\left( Z_{r} \right)} - {{Min}\left( Z_{r} \right)}}{R_{1} + {{Max}\left( Z_{r} \right)}} = {\frac{\frac{\omega^{2}M^{2}}{R_{2}R_{1}}}{1 + \frac{\omega^{2}M^{2}}{R_{2}R_{1}}} = \frac{k^{2}Q_{1}Q_{2}}{{4\pi^{2}} + {k^{2}Q_{1}Q_{2}}}}}} & (3)\end{matrix}$

Where R₁ is the equivalent series resistance (ESR) of the reader'santenna resonant circuit, i.e., the sum of Resistor 501 and Resistor503, Q₁ and Q₂ are the Q-factors of antenna resonant circuits of thereader interface and the tag interface, respectively. The maximum andminimum signal strengths are achieved when H is equal to 1 and 0,respectively.

The dimension of antennas mainly affects the coupling coefficient kbetween the antennas of the reader and the tag. For a given distancebetween antennas, smaller antennas lead to a lower coupling coefficient.According to Equation (3), a low coupling coefficient will lower thetag->reader signal strength H, which may cause the NFC reader interfaceto drop the frame due to low SNR (signal-to-noise ratio).

Based on the above analysis, there are a few methods to deal with thelow coupling coefficient caused by small antennas:

(1) Improving the Q-factor of the reader's antenna resonant circuit, Q₁.According to Equation (3), increasing Q₁ would improve the tag->readersignal strength H. Q₁ is the combined Q-factor of the entire antennaresonant circuit, which is determined collaboratively by the Q-factor ofthe antenna, the ESR of matching circuit and the antenna driver, andetc.

(2) Improving the Q-factor of the tag's antenna resonant circuit, Q₂.According to Equation (3), increasing Q₂ would improve the tag->readersignal strength H. Q₂ is the combined Q-factor of the entire antennaresonant circuit, which is determined collaboratively by the Q-factor ofthe antenna, the ESR of matching circuit, and etc.

(3) Adjusting the resistance switching range of load R_(L). According toEquation (3), increasing the range that R_(L) could switch would improvethe tag->reader signal strength H. In most cases, the maximum andminimum values of R_(L) are determined by the characteristics of theload switch in the load modulator. To improve H, the load switch has asmall input capacitance, high isolation, and a low insertion loss.

From the above analysis, the common solution for improving wirelesspower transfer efficiency and communication performance when using smallantennas, is to improve the Q-factors of both reader and tag interfaces.However, for communication, having high Q-factor will also decreaseavailable communication bandwidth, lowering communication data rate.

The present invention discloses a method for using multiple antennaresonant circuits with different Qs to satisfy the contradictingrequirements. The invention uses low Q antenna resonant circuits for thereader->tag link communication, and uses high Q antenna resonantcircuits for the tag->reader link communication and wireless powertransfer. Because a reader usually has much higher processing power thana tag due to its abundant energy, the NFC reader can performsophisticated signal processing on a received signal to mitigate thedistortion caused by the low bandwidth.

NFC Reader in One Embodiment of the Invention

FIG. 6 shows the architecture of a NFC reader in one embodiment of theinvention. Antenna 601 is a high Q (Q>100) antenna, which creates anoscillating magnetic field and receives a NFC signal from a NFC tag. TheBi-Q Matching Circuit 602 transforms the impedance of the Antenna 601 toa proper level, and has two working modes: a low-Q mode and a high-Qmode. These two modes adjust the Q-factor of the antenna resonantcircuits to a low Q (Q≤25) and a high Q (Q≥50) to optimize performance,respectively. Modulator 604 creates modulated NFC signal superimposed ona 13.56 MHz carrier according to the NFC data to be transmitted, andfeeds the signal to Antenna Driver 603. The driver 603 has low outputimpedance for improving power efficiency, and drives the Antenna 601through the Bi-Q Matching Circuit 602. Demodulator 609 measures andtracks the current flowed through the antenna, and demodulates thesignal. MCU 607 controls and oversees the operation of the entire NFCreader. Its tasks include packing and de-packing NFC frames, verifyingdata, communicating with external devices via a bus, controlling variouson-chip components, and etc. Bus Interface 605 is the communication portfor exchanging data with external devices, and is usually in the form ofUART, SPI, or I2C. FIFO 606 serves as the bridge between Bus Interface605 and MCU 607. Clock System 608 generates all clocks used on the NFCreader, including the 13.56 MHz carrier frequency. The On-chip PowerSupply provides power for all on-chip components.

Different from a conventional NFC reader design, the disclosed NFCreader design has an antenna resonant circuit with two working modes,i.e., the high-Q mode and the low-Q mode. When working in the high-Qmode, the antenna resonant circuit has a high Q-factor but a lowbandwidth. The 13.56 MHz carrier signal could be emitted at a very lowloss, which is well suited for wireless power transfer. When working inthe low-Q mode, the antenna resonant circuit has a low Q-factor but ahigh bandwidth, which is especially suitable for NFC signaltransmission. These two modes could be switched in real time by the MCU.The NFC reader design is completely compatible with current NFCstandards.

FIG. 7 shows one implementation of the reader's antenna resonantcircuit. Antenna Driver 701, which is Antenna Driver 603 shown in FIG.6, is a low output impedance (<5 Ohm) RF power amplifier. AntennaMatching Circuit 702, Q Adjustment Resistor 703, and RF Switch 704constitute the Bi-Q Matching Circuit 602 shown in FIG. 6. AntennaMatching Circuit 702 transforms the impedance of the Antenna 705 to asuitable level for power and efficiency control. As Antenna MatchingCircuit 702 introduces loss, it is as simple as possible to improveefficiency. FIG. 7 shows the Antenna Matching Circuit 702 as a PImatching circuit, however, any other simple forms could be employed aswell, like L-pad and single capacitor. The Antenna Matching Circuit 702has an insertion loss smaller than or equal to 1 db to improve theQ-factor of the antenna resonant circuit in high-Q mode. Q AdjustmentResistor 703, together with RF Switch 704, controls the working mode ofthe antenna resonant circuit. To be specific, when RF Switch 704 isopen, the antenna resonant circuit is in the high-Q mode, and viceversa. The actual resistance of Q Adjustment Resistor 703 is computedaccording to the characteristics of other components in the circuit.

Antenna 705, which is Antenna 601 shown in FIG. 6, is designed toachieve optimum wireless power transfer efficiency and communicationperformance. First, Antenna 705 resonates at around 13.56 MHz. Due tothe limitation of practical tuning capacitors, the existence ofparasitic capacitance, and the mutual inductance caused by the tagantenna, the inductance of Antenna 705 cannot be too large. On the otherhand, too small inductance leads to a low Q and low efficiency.Therefore, the optimal inductance value is within 1 uH to 10 uH. Second,Antenna 705 has a sufficiently high Q. This could be achieved by usingwider and thicker antenna tracks, low impedance antenna wires, and lowRF loss base materials. Third, Antenna 705 has a size that is proper forproviding sufficient coupling. For typical applications, an area in therange of 100 mm² to 5000 mm² would be sufficient.

The disclosed NFC reader design adjusts its working mode in real time inaccordance to its current state. It spends most of the time in thehigh-Q mode for supporting high-efficient wireless power transfer. FIG.8 shows the state machine transition of the disclosed NFC reader design.The initial working mode after device powering up is the high-Q mode. Atime t1 after the power-up event, MCU activates the RF interface byenabling the modulator, the antenna driver, along with a few otherperipheral components. It then generates the unmodulated 13.56 MHzcarrier, which is radiated via the RF interface. This carrier generatesan oscillating magnetic field around the antenna, which can be utilizedto power nearby NFC tags. The NFC reader has to wait at least t2 timeafter RF interface activation before it can transmit any data, whichensures nearby NFC tags harvest sufficient energy for proper operation.Then the NFC reader switches to the low-Q mode to start datatransmission, and switches back to the high-Q mode immediately aftertransmission. It then waits for t3 time after transmission for tagresponse. If there is a response, the NFC reader has to wait for t6 timein the high-Q mode before entering the low-Q mode for transmitting thenext data frame. Otherwise the NFC reader switches to another modulationscheme and attempts transmission again after t5 time, in order to detecttags that support different modulation schemes. If the NFC reader hasattempted all modulations but has still received no response, itdetermines that there is no tag nearby and deactivates its RF interfaceto save energy. The NFC reader waits t4 time and repeats all aboveprocess to detect new tags. The value of t1 guarantees that all thecomponents on the reader are properly prepared for the subsequent RFcommunications. The time t4 is determined according to a maximum allowedtag detection delay and an energy budget, and has a value between 0.1 sto 1 s. The time t2 is to ensure that the tag receives sufficient energyfrom the reader to perform the subsequent communications, and is atleast 5 ms. The values of t3, t5, and t6 may respectively be the same asthe Frame delay time PCD to PICC, Request Guard Time, and Frame delaytime PICC to PCD as defined in the ISO14443-3 standard.

First Embodiment of NFC Tag

FIG. 9 shows the architecture of a NFC tag interface in one embodimentof the invention. Antenna 901 is a loop antenna with a high Q, whichreceives the wireless power and NFC signals transmitted by a nearby NFCreader, as well as transmits NFC signals. Bi-Q Matching Circuit 902adjusts the impedance of Antenna 901 to a proper value. It has twoworking modes, i.e., a high-Q mode and a low-Q mode, which can tune theQ-factor of the antenna resonant circuit to a high (Q≥50) and low (Q≤25)value, respectively. Demodulator 903 demodulates the NFC signalsreceived by Antenna 901. Load Modulator 905 modulates the load toAntenna 901 in order to transmit the signal. Bus Interface 904 connectsto external devices via a system data bus, and is used for exchangingNFC data and configuring the NFC tag interface. Rectifier and RegulatorCircuit 906 converts the RF energy that Antenna 901 harvested to aregulated DC power, which could be used for powering the tag interface,and external devices via Energy Harvest Interface 907. A power switch ispresent in the Rectifier and Regulator Circuit 906 to control the energypath to the Energy Harvest Interface 907.

Unlike a conventional NFC tag design, the disclosed NFC tag design has abi-Q antenna resonant circuit, which can work in the high-Q mode and thelow-Q mode. When working in the high-Q mode, the antenna resonantcircuit has a high Q-factor but a low bandwidth. The 13.56 MHz carriersignal could be received at a very low loss, which is well suited forwireless power reception (a.k.a., energy harvesting). When working inthe low-Q mode, the antenna resonant circuit has low a Q-factor but ahigh bandwidth, which is especially suitable for NFC signal reception.These two modes could be switched in real time. The disclosed NFC tagdesign is completely compatible with current NFC standard.

FIG. 10 shows one implementation of the tag's antenna resonant circuit.Antenna 1005 (i.e., Antenna 901 in FIG. 9) and Antenna Matching Circuit1004 comprise the antenna resonant circuit. As Antenna Matching Circuit1004 introduces loss, it is as simple as possible to improve efficiency.FIG. 10 shows the Antenna Matching Circuit 1004 as a single parallelcapacitor. However, any other simple forms could be employed as well,like the L-pad and PI/T matching network. Resistor 1006 and RF Switch1007 work as a Q-factor adjustment circuit. The on and off states of RFSwitch 1007 correspond to the low-Q and high-Q modes, respectively. Thevalue of Resistor 1006 is determined by the characteristics of Antenna1005, Antenna Matching Circuit 1004, and RF Switch 1007, so that the Qvalue can be control to be no larger than 25 when RF Switch 1007 isclosed. Antenna Matching Circuit 1004, Resistor 1006 and RF Switch 1007constitute the Bi-Q Matching Circuit 902 shown in FIG. 9. Resistor 1003and RF Switch 1002 comprise the load modulator (i.e., Load Modulator 905in FIG. 9). Rectifier 1001 converts the RF energy that Antenna 1005receives to DC power, and feeds the power to a regulator circuit viaPower Switch 1008, which controls the connection of the external load tothe rectifier. Rectifier 1001, Power Switch 1008, and the connectedvoltage regulator constitute the Rectifier and Regulator Circuit 906.Rectifier 1001 also connects with the demodulator.

Antenna 1005 is designed to reach optimum wireless power receptionefficiency and communication performance. First, Antenna 1005 resonatesat around 13.56 MHz. Due to the limitation of practical tuningcapacitors, the existence of parasitic capacitance, and the mutualinductance caused by tag antenna, the inductance of Antenna 1005 cannotbe too large. On the other hand, too small inductance leads to a low Qand low efficiency. Therefore, the optimal inductance value is within 1uH to 10 uH. Second, Antenna 1005 has a sufficiently high Q. This couldbe achieved by using wider and thicker antenna tracks, low impedanceantenna wires, and low RF loss base materials. Third, Antenna 1005 has asize that is proper for providing sufficient coupling. For typicalapplications, an area in the range of 100 mm² to 5000 mm² would besufficient. A smaller Antenna is still able to communicate, but withlower performance (shorter distance, lower data rate, etc.).

Load modulator maximizes the switching range of load impedance toimprove the communication performance of the tag->reader link, whichcould be achieved with a high isolation and a low on-resistance of RFSwitch 1002. Typically an isolation value higher than 10 KOhm, and anon-resistance lower than 50 Ohm should be sufficient. RF Switch 1002also has a sufficiently high power rating to handle the high powerdissipation when switched. If the power rating is too low, Resistor 1003is used for limiting the power dissipated on RF Switch 1002, however, atthe expense of lowering the switching range. Due to the high frequencyof the NFC subcarrier signal (848 KHz) that is to be transmitted, RFSwitch 1002 has a switching speed higher than 1 MHz.

The disclosed NFC tag design adjusts its working mode in real time inaccordance to its current state. It spends most of the time in thehigh-Q mode for supporting high-efficient wireless power reception. FIG.11 shows the state machine transition of the disclosed NFC tag design.The initial working mode after device powering up is the low-Q mode.After powering up, the tag stays in low power mode and continuouslydetects for NFC 13.56 MHz magnetic field. If it successfully detects anearby field, it then measures if the field has strength stronger thanA_(t). If it does, the tag switches to the high-Q mode for wirelesspower reception and connects the load to a rectifier. Otherwise, itstays in low Q mode and disconnects the load. This mechanism preventsthe wireless power reception from disrupting the communication when thefield is too weak. Then the tag detects if there is a modulated signaltransmitted by a nearby reader. If there is none, it stays in thecurrent working mode, goes back to field detection, and repeats theabove processes. If there is a modulated signal detected, it immediatelyswitches to the low-Q mode, and receives the signal. After signalreception, it switches back to high Q mode, and transmits responsesignal using load modulation within t1 time. After transmission, the tagdetects if the field is still present, and repeats all above processesif it does. Otherwise it switches back to the low-Q mode and repeatfield detection. The values of t1 may be the same as the Frame delaytime PICC to PCD defined in the ISO14443-3 standard.

A_(t) is set according to the actual field strength when the NFC tag isclose to the NFC reader. It has hysteresis, i.e., its value when a loadis connected is lower than that when the load is unconnected. Thisprevents oscillating. The hysteresis value is determined according tothe intended system load.

Because the modulation detection may be performed in the high-Q mode andthe actual signal reception is performed in the low-Q mode, if the datarate is high, several modulation symbols may be missed during modetransition. Therefore, this disclosed NFC tag design can only supportNFC protocols with a lower data rate.

Sometimes NFC tag interfaces are battery powered (active tags), and donot need wireless power reception function. The state machine transitionof these tag interfaces is shown in FIG. 15. The initial working modeafter device powering up is the low-Q mode. After powering up, the tagstays in low power mode and continuously detects for NFC 13.56 MHzmagnetic field. If it successfully detected a nearby field, it thendetects if there is a modulated signal transmitted by the nearby reader.If there was none, it goes back to field detection, and repeats theabove processes. If there is a modulated signal detected, it receivesthe signal. After signal reception, it switches to the high-Q mode, andtransmits a response signal using load modulation within t1 time. Aftertransmission, the tag goes back to the low-Q mode and detects if thefield is still present, and repeats all above processes. The values oft1 may be the same as the Frame delay time PICC to PCD defined in theISO14443-3 standard.

Second Embodiment of NFC Tag

FIG. 12 shows the architecture of the NFC tag interface in anotherembodiment of the invention. The disclosed NFC tag design contains twoantennas, Antenna 1201 and Antenna 1205, and two antenna resonantcircuits. Antenna 1201 and Antenna Matching Circuit 1202 comprise thefirst antenna resonant circuit with a low Q (Q≤25), which is utilized toreceive the NFC signal transmitted by nearby NFC readers. Antenna 1205and Antenna Matching Circuit 1206 comprise the second antenna resonantcircuit with a high Q (Q≥50), which is used to harvest nearby RF energyand perform load modulation. Demodulator 1203 demodulates the signalreceived by Antenna 1201, and sends the data to Bus Interface 1204,which connects with external devices via a data bus. Load Modulator 1208modulates the load connected to Antenna 1205 according to the datareceived from the data bus. Bus Interface 1204 serves as the dataexchanging hub for Demodulator 1203, Load Modulator 1208, and externaldevices. Rectifier and Regulator Circuit 1207 converts the RF energythat Antenna 1205 harvested to regulated DC power, which could be usedfor powering the tag interface, and external devices via Energy HarvestInterface 1209. A power switch is present in the Rectifier and RegulatorCircuit 1207 to control the energy path to the Energy Harvest Interface1209.

Unlike the conventional NFC tag design, the disclosed NFC tag design hastwo antenna resonant circuits that have a high Q and a low Q,respectively. With the low Q, the first antenna resonant circuitprovides a high NFC reception bandwidth. With the high Q, the secondantenna resonant circuit provides exceptional wireless power receptionefficiency. The two antenna resonant circuits work together, whichoffers both high communication and wireless power reception performance.This disclosed NFC tag design can support all standard data rates, andis completely compatible with current NFC standards.

FIG. 13 shows one implementation of the antenna resonant circuits.Antenna 1310 (i.e., Antenna 1201 in FIG. 12), Capacitor 1311, andResistor 1309 comprise the first antenna resonant circuit. Capacitor1311 serves as the tuning capacitor to control the impedance of Antenna1310 at 13.56 MHz. The purpose of Resistor 1309 is to lower the Q of thefirst antenna resonant circuit to be smaller than or equal to 25 if theQ of Antenna 1310 is too high. Capacitor 1311 and Resistor 1309constitute the Antenna Matching Circuit 1202 shown in FIG. 12. The NFCdemodulator connects with the first antenna resonant circuit. Antenna1306 (i.e., Antenna 1205 in FIG. 12) and Antenna Matching Circuit 1304(i.e., Antenna Matching Circuit 1206 in FIG. 12) comprise the secondantenna resonant circuit. FIG. 13 shows the Antenna Matching Circuit1304 as a single parallel capacitor—however, any other simple form couldbe employed as well, like L-pad and PI/T matching network. The Q of thesecond antenna resonant circuit is above 50. Resistor 1303 and RF Switch1302 comprise the load modulator (i.e., Load Modulator 1208 in FIG. 12).Rectifier 1301 converts the RF energy that Antenna 1005 received to DCpower, and feeds the power to a regulator circuit via Power Switch 1308,which controls the connection of the external load to the rectifier.Rectifier 1301, Power Switch 1308, and the connected voltage regulatorconstitute the Rectifier and Regulator Circuit 1207.

As only the second antenna resonant circuit can harvest the energy (theenergy received by the first antenna resonant circuit is converted toheat), to improve wireless power reception efficiency, the energy thatis received by the first antenna resonant circuit is sufficiently low.To be specific, the received signal strength is as low as possible, buthigher than the reception sensitivity of the demodulator. There are afew methods to lower the reception voltage. First, lowering the Q of thefirst antenna resonant circuit at 13.56 MHz. This makes the circuit lesssensitive to a 13.56 MHz signal. It could be done by increasing thevalue of Resistor 1309, or tuning the resonant frequency away from 13.56MHz by carefully choosing the value of Capacitor 1311. Second,decreasing the inductance of the antenna. This reduces the mutualinductance between antennas of the reader and the first antenna resonantcircuit, which decreases induced voltage. It could be achieved by usingantennas with less loops or a smaller encompassed area. Third,decreasing the coupling between the antennas of the reader and the firstantenna resonant circuit. This could be done by decreasing the area ofthe antenna, or moving the antenna away.

Load modulator maximizes the switching range of load impedance toimprove the communication performance of the tag->reader link, whichcould be achieved with a high isolation and a low on-resistance of RFSwitch 1302. Typically an isolation value higher than 10 KOhm, and anon-resistance lower than 50 Ohm should be sufficient. RF Switch 1302also has sufficiently high power rating to handle the high powerdissipation when switched. If the power rating is too low, Resistor 1303is used for limiting the power dissipated on RF Switch 1302, however, atthe expense of lowering the switching range. Due to the high frequencyof the NFC subcarrier signal (848 KHz) that is to be transmitted, RFSwitch 1302 has a switching speed higher than 1 MHz.

The second antenna resonant circuit is designed to reach optimumwireless power transfer efficiency and communication performance. First,the second antenna resonant circuit resonates at around 13.56 MHz. Dueto the limitation of practical tuning capacitors, the existence ofparasitic capacitance, and the mutual inductance caused by tag antenna,the inductance of Antenna 1306 cannot be too large. On the other hand,too small inductance leads to a low Q and low efficiency. Therefore, theoptimal inductance value is within 1 uH to 10 uH. Second, the secondantenna resonant circuit has a sufficiently high Q. This means high Qfor Antenna 1306 and low loss for Antenna Matching Circuit 1304. Highantenna Q could be achieved by using wider and thicker antenna tracks,low impedance antenna wires, and low RF loss base materials. Lowmatching circuit loss could be achieved by using simple matchingtopology, as more components mean more loss. Third, Antenna 1306 has asize that is proper for providing sufficient coupling. For typicalapplications, an area that is in the range of 100 mm² and 5000 mm² wouldbe sufficient. Fourth, the output impedance of the second antennamatching circuit matches that of the load. This could be done byadjusting the impedance transformation of Antenna Matching Circuit 1304.

FIG. 14 shows the state machine transition of the disclosed NFC tagdesign. After powering up, the tag works in a low power mode andcontinuously detects for a RF field. If a RF field is successfullydetected, it tests if the field strength is higher than A_(t). If itdoes, the tag connects the load to the rectifier and powers the load,otherwise it disconnects the load. This mechanism prevents the wirelesspower reception from disrupting the communication when the field is tooweak. Then the tag detects if there is modulated signal transmitted by anearby reader. If there is none, it goes back to field detection andrepeats the above processes. If there is a modulated signal detected, itreceives the signal. After signal reception, it transmits a responsesignal using load modulation within t1 time. After transmission, the tagdetects if the field is still present, and repeats all above process ifit does. A_(t) is set according to the actual field strength when theNFC tag is close to the NFC reader. It has hysteresis, i.e., its valuewhen load is connected is lower than that when the load is unconnected.This prevents oscillating. The hysteresis value is determined accordingto the intended system load. The values of t1 may be the same as theFrame delay time PICC to PCD defined in the ISO14443-3 standard.

The invention claimed is:
 1. A device for concurrent near-fieldcommunication (NFC) and wireless power reception (WPR) using a magneticfield, comprising: a low-Q antenna resonant circuit configured toperform the NFC, and including a first antenna for magnetic flux of themagnetic field to flow therethrough, to thereby receive a NFC signal forthe NFC, and a first antenna matching circuit that is connected to thefirst antenna and is so configured that a quality factor (Q-factor) ofthe low-Q antenna resonant circuit is no higher than 25; and a high-Qantenna resonant circuit configured to perform the WPR, and including asecond antenna for the magnetic flux of the magnetic field to flowtherethrough, to thereby receive wireless power for the WPR, and asecond antenna matching circuit that is connected to the second antennaand is so configured that the Q-factor of the high-Q antenna resonantcircuit is no lower than 50, wherein the low-Q and high-Q antennaresonant circuits are separate from each other, the high-Q antennaresonant circuit operates to perform WPR, responsive to strength of themagnetic field being larger than a predetermined threshold, and thehigh-Q antenna resonant circuit and the low-Q antenna resonant circuitoperate to perform NFC, responsive to the strength of the magnetic fieldbeing no larger than the predetermined threshold.
 2. The device of claim1, wherein the high-Q antenna resonant circuit is configured to performdata transmission.
 3. The device of claim 1, further comprising: ademodulator connected to the first antenna matching circuit andconfigured to demodulate the NFC signal received by the first antenna; adata interface configured to transmit the demodulated NFC signal to afirst external device; a rectifier-and-regulator connected to the secondantenna matching circuit and configured to convert the wireless powerreceived by the second antenna to regulated DC (direct current) energy;and an energy harvest interface configured to supply the regulated DCenergy to a second external device.
 4. The device of claim 3, whereinthe second antenna is further configured to transmit another NFC signal,and has a load connected thereto; the device further comprises a loadmodulator configured to modulate an impedance of the load fortransmitting the another NFC signal; and the data interface is furtherconfigured to receive data corresponding to the another NFC signal fromthe first external device, and to send the received data to the loadmodulator.
 5. The device of claim 4, wherein the load modulator includesa radio frequency (RF) switch that has an isolation value larger than 10KOhm, an on-resistance value smaller than 50 Ohm, and a switching speedlarger than 1 MHz.
 6. The device of claim 3, wherein therectifier-and-regulator includes a switch for controlling a connectionfrom the rectifier-and-regulator to the energy harvest interface.
 7. Thedevice of claim 1, wherein each of the first and second antenna matchingcircuits utilizes an impedance transformation topology, and has aninsertion loss smaller than 1 db.
 8. The device of claim 1, wherein thefirst antenna has at least one of an inductance value smaller than 4 uH,a Q value smaller than 50, and a coupling coefficient, with respect toan external antenna coupled thereto, is smaller than 0.1.
 9. The deviceof claim 1, wherein the second antenna has an inductance in a range of 1uH to 10 uH, an area in a range of 100 mm² to 5000 mm², and a Q valuehigher than 50.